Significance of Impurity Mineral Identification in the Value Addition of Kaolin – A Case Study with Reference to an Acidic Kaolin from India

Abstract

Kaolin or china clay is a versatile industrial mineral with wide technological applications and is abundantly available in India. The major mineral in kaolin is kaolinite (Al2O3.2SiO2.2H2O). The common ancillary / impurity minerals occurring with kaolin include parent rocks like feldspar and mica, quartz, ferruginous, titanoferrous and carbonaceous materials. The most deleterious impurities in kaolin are iron minerals which imparts colour to the white kaolin. Iron exists as oxides, hydroxides, oxy hydroxides, sulphides and carbonates along with iron stained quartz/anatase and mica in kaolin. Kaolin finds extensive applications in paper, paint, rubber, ceramics, plastics etc. One of the highest value additions for kaolin is as pigment in paper and paint industries. The optical properties are important for pigment applications and removal of the iron impurity is very important to improve this property. Extensive research has been carried out on the nature of iron impurities present in kaolin, which leads to the conclusion that iron is present as a part of the kaolinite or ancillary mineral (mica or titania) structure, which can be termed as “structural iron” or as independent iron minerals such as oxides, hydroxides, oxyhydroxides, sulphides and carbonates, which can be termed as “free iron” [1]. The present paper discusses the iron speciation studies carried out on a typical china clay sample collected from Koraput district of Orissa State in the Union of India. Studies have shown that the major impurity mineral species is in “pyritic” (Iron sulphide) form along with other hydroxides, oxyhydoxides and oxides of Iron. Presence of limonite is also observed in the sample. The identification/quantification of the impurity minerals have played a crucial role in the selection / modification and sequentialisation of beneficiation processes and subsequent processing studies have shown that the sample can be value added to ceramic grade.